Plant lipids have a variety of industrial and nutritional uses and are central to plant membrane function and climatic adaptation. These lipids represent a vast array of chemical structures, and these structures determine the physiological and industrial properties of the lipid. Many of these structures result either directly or indirectly from metabolic processes that alter the degree of unsaturation of the lipid. Different metabolic regimes in different plants produce these altered lipids, and either domestication of exotic plant species or modification of agronomically adapted species is usually required to produce economically large amounts of the desired lipid.
There are serious limitations to using mutagenesis to alter fatty acid composition and content. Screens will rarely uncover mutations that a) result in a dominant (“gain-of-function”) phenotype, b) are in genes that are essential for plant growth, and c) are in an enzyme that is not rate-limiting and that is encoded by more than one gene. In cases where desired phenotypes are available in mutant corn lines, their introgression into elite lines by traditional breeding techniques is slow and expensive, since the desired oil compositions are likely the result of several recessive genes.
Recent molecular and cellular biology techniques offer the potential for overcoming some of the limitations of the mutagenesis approach, including the need for extensive breeding. Some of the particularly useful technologies are seed-specific expression of foreign genes in transgenic plants [see Goldberg et al (1989) Cell 56:149-160], and the use of antisense RNA to inhibit plant target genes in a dominant and tissue-specific manner [see van der Krol et al (1988) Gene 72:45-50]. Other advances include the transfer of foreign genes into elite commercial varieties of commercial oilcrops, such as soybean [Ghee et al (1989) Plant Physiol. 91:1212-1218; Christou et al (1989) Proc. Natl. Acad. Sci. U.S.A. 86:7500-7504; Hinchee et al (1988) Bio/Technology 6:915-922; EPO publication 0 301 749 A2], rapeseed [De Block et al (1989) Plant Physiol. 91:694-701], and sunflower [Everett et al (1987) Bio/Technology 5:1201-1204], and the use of genes as restriction fragment length polymorphism (RFLP) markers in a breeding program, which makes introgression of recessive traits into elite lines rapid and less expensive [Tanksley et al (1989) Bio/Technology 7:257-264]. However, application of each of these technologies requires identification and isolation of commercially-important genes.
Most free fatty acids become esterified to coenzyme A (CoA), to yield acyl-CoAs. These molecules are then substrates for glycerolipid synthesis in the endoplasmic reticulum of the cell, where phosphatidic acid and diacylglycerol (DAG) are produced. Either of these metabolic intermediates may be directed to membrane phospholipids (e.g., phosphatidylglycerol, phosphatidylethanolamine, phosphatidylcholine) or DAG may be directed to form triacylglycerols (TAGs), the primary storage reserve of lipids in eukaryotic cells.
Diacylglycerol acyltransferase (“DGAT”) is an integral membrane protein that catalyzes the final enzymatic step in the production of triacylglycerols in plants, fungi and mammals. This enzyme is responsible for transferring an acyl group from acyl-coenzyme-A to the sn-3 position of 1,2-diacylglycerol (“DAG”) to form triacylglycerol (“TAG”). DGAT is associated with membrane and lipid body fractions in plants and fungi, particularly, in oilseeds where it contributes to the storage of carbon used as energy reserves. TAG is believed to be an important chemical for storage of energy in cells. DGAT is known to regulate TAG structure an direct TAG synthesis. Furthermore, it is known that the DGAT reaction is specific for oil synthesis.
TAG is the primary component of vegetable oil in plants. It is used by the seed as a stored form of energy to be used during seed germination.
Two different families of DGAT proteins have been identified. The first family of DGAT proteins (“DGAT1”) is related to the acyl-coenzyme A:cholesterol acyltransferase (“ACAT”) and has been described in U.S. Pat. Nos. 6,100,077 and 6,344,548. A second family of DGAT proteins (“DGAT2”) is unrelated to the DGAT1 family and is described in PCT Patent Publication WO 2004/011671 published Feb. 5, 2004. Other references to DGAT genes and their use in plants include PCT Publication Nos. WO2004/011,671, WO1998/055,631, and WO2000/001,713, and US Patent Publication No. 20030115632.
Applicants' Assignee's published patent application US 2006-0094088 describes genes for DGATs of plants and fungi and their use is in modifying levels of polyunsaturated fatty acids (“PUFAs”) in edible oils.
Applicants' Assignee's published PCT application WO 2005/003322 describes the cloning of phosphatidylcholine diacylglycerol acyltransferase and DGAT2 for altering PUFA and oil content in oleaginous yeast.